Sorbent formulation for removal of mercury from flue gas

ABSTRACT

Methods and systems for reducing mercury emissions from fluid streams having a high concentration of sulfur oxide species are provided herein. In embodiments, mercury is removed from flue gas streams containing sulfur trioxide (SO 3 ) by injecting a dry admixture of a porous mercury adsorptive material and at least one dry agent into the flue gas stream.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of priority to U.S. Provisional PatentApplication No. 61/246,402 filed on Sep. 28, 2009 entitled“Sulfur-Tolerant Sorbent Compositions For Mercury Removal From FlueGas,” U.S. Provisional Patent Application No. 61/246,398 filed on Sep.28, 2009 entitled “Novel Carbon Based Sorbents for High SO₃Applications,” and U.S. Provisional Patent Application No. 61/349,332filed on May 28, 2010 entitled “Sulfur-Tolerant Sorbent Compositions ForMercury Removal From Flue Gas,” the entire contents of which are herebyincorporated by reference.

GOVERNMENT INTERESTS

Not applicable

PARTIES TO A JOINT RESEARCH AGREEMENT

Not applicable

INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

Not applicable

BACKGROUND

Mercury is a known environmental hazard and leads to health problems forboth humans and non-human animal species. Approximately 50 tons per yearare released into the atmosphere in the United States, and a significantfraction of the release comes from emissions from coal burningfacilities such as electric utilities. To safeguard the health of thepublic and to protect the environment, the utility industry iscontinuing to develop, test, and implement systems to reduce the levelof mercury emissions from its plants. In the combustion of carbonaceousmaterials, it is desirable to have a process wherein mercury and otherundesirable compounds are captured and retained after the combustionphase so that they are not released into the atmosphere.

One of the most promising solutions for mercury removal from flue gas isActivated Carbon Injection (ACI). Activated carbon is a highly porous,non-toxic, readily available material that has a high affinity formercury vapor. This technology is already established for use withmunicipal incinerators. Although the ACI technology is effective formercury removal, the short contact time between the activated carbon andthe flue gas stream results in an inefficient use of the full adsorptioncapacity of the activated carbon. Mercury is adsorbed while the carbonis conveyed in the flue gas stream along with fly ash from the boiler.The carbon and fly ash are then removed by a particulate capture devicesuch as an Electrostatic Precipitator (ESP) or baghouse.

In flue gas streams containing high concentrations of sulfur oxides,mercury removal by the injection of activated carbon is oftencompromised by the preferential adsorption and retention of the sulfurcompounds such as sulfur trioxide, which are strongly adsorbed by carbonsorbents. The concentration of sulfur dioxide relative to mercury in atypical flue gas stream can be one million to one or higher, and theconcentration of sulfur trioxide in such flue gas streams are typicallyone thousand to one. For example, high sulfur flue gas streams maycontain from about 500 parts-per million per volume (ppmv) to about 2500ppmv or more sulfur dioxide and from about 2 ppmv to about 20 ppmv ormore sulfur trioxide. Water vapor in the flue gas further compounds theproblem by combining with sulfur trioxide to form sulfuric acid in thepores of the carbon, effectively excluding the adsorption and removal ofmercury. For utilities that burn bituminous coals or mixtures ofbituminous coals with lower rank sub-bituminous coals, the presence ofhigh levels of sulfur oxides, especially sulfur trioxide, can be asignificant concern.

In addition to sulfur oxides that form during coal combustion, sulfurtrioxide may also be produced inadvertently in selective catalyticreduction (SCR) processes downstream of the boiler for controlling NOemissions, or it may be added to the flue gas to enhance the performanceof ESP devices used to capture the fly ash. Regardless of its origins,sulfur trioxide may have unintended consequences beyond its interferencewith mercury removal that affect the performance and profitability ofthe power plant. These consequences include corrosion of systemcomponents and unwanted increases in plume visibility and duration upondischarge from the stack.

To prevent the interference of sulfur oxides with mercury removal by theinjected mercury sorbent, a number of prior art solutions have beenproposed wherein gross reductions in total sulfur oxide levels areachieved in the gas phase. Nearly all of these solutions rely upon bulkinjections of alkaline or other reactive agents into the flue gas toreact chemically with the sulfur oxides, forming salt particulates inthe gas phase which do not usually interfere with mercury adsorption bythe sorbent. In some cases, the agent is injected as a dry solid (DrySorbent Injection (DSI)), while in other methods an aqueous solution ofthe agent is injected, which rapidly devolatizes at the temperature ofinjection to form a very fine, dry powder with even higher reactivitytoward sulfur oxides in the duct. For example, trona, anaturally-occurring mixture of sodium carbonate and sodium bicarbonate,is a commercially-available material found to be effective incontrolling sulfur oxides when injected into flue gas streams as a dryreactant.

Other alkaline agents, such as calcium oxide (lime), calcium hydroxide(slaked lime), calcium carbonate (limestone), magnesium carbonate(dolomite), magnesium hydroxide, magnesium oxide, and sodium carbonateare also utilized to control sulfur oxide emissions. Solution injectionof such agents is represented by Codan's SBS Injection™ technology,which uses an aqueous solution of the chemical reductants sodiumbisulfite and sulfite, and is more selective and effective for sulfurtrioxide removal. Alternatively, solutions of sodium carbonate,bicarbonate, or hydroxide or thiosulfate can also be used. However, allof these materials and methods suffer disadvantages in that relativelylarge amounts of the agent must be used for effective control and, moreimportantly, separate injection systems must be installed independent ofmercury sorbent injection, adding cost and complexity to theirapplication. In the case of alkali-based agents, a further disadvantageis found in the negative impact of such materials on the properties ofthe fly ash collected for subsequent sale to the cement and concreteindustry. Although this disadvantage is avoided by using alkalineearth-based agents, these agents generally impart an unwanted increasein resistivity to the ESP, while the alkali-based agents usually haveminimal impact on ESP operation.

Where alkaline or other SO_(x) reactive agents have been incorporatedinto the pore structure of the sorbents themselves, the intent has beenuniformly the removal of the sulfur compounds and not the removal ofmercury in the presence of such compounds. Numerous other examples ofactivated carbons and other sorbents that incorporate SO_(x)-reactivematerials within the body of the sorbent have been reported, but noneappear to advance the art of mercury removal since they are neitherdirected to that purpose nor are they likely to offer a preferredsolution since major portions of the pore structure available formercury adsorption are configured preferentially for sulfur oxideremoval.

There is a need to provide dry sorbent compositions for mercury removalin flue gas streams containing high concentrations of sulfur oxides,especially sulfur trioxide, that do not depend on the independentinjection of alkaline or other reactive agents elsewhere in the systemfor effective mercury removal, and are inherently effective in a singleinjection mode. Where such alkaline or reactive agents are used as partof the dry sorbent compositions, there is a further need to limit theimpact of these agents on balance-of-plant operations by using only whatmay be necessary to enhance mercury removal locally at the point ofsorbent injection, as well as to avoid incorporation within the body ofthe porous sorbent to afford increased opportunity for mercury removal.Where independent injection of said alkaline or reactive agents may yetbe necessary, there is also a need to reduce the amount of such agentsthat might otherwise be used, consistent with effective mercury removaland marginal impacts on balance of plant issues.

SUMMARY OF THE INVENTION

Various embodiments are directed to a composition including a a porousabsorptive material, a source of halogen and a source of nitrogenwherein said source of nitrogen has an oxidation state of −3 and saidcomposition having a particle size of less than 12 μm. In someembodiments, the source of nitrogen may be selected from the groupconsisting of ammonium, ammonia, amines, amides, imines, quaternaryammonium and the like, and in other embodiments, the composition mayhave a mean particle diameter of less than about 7 μm. In still otherembodiments, the at least one agent is ammonium bromide. The porousmercury adsorptive material may be any of a carbonaceous char, activatedcarbon, reactivated carbon, zeolite, silica, silica gel, alumina clay,or a combination thereof. In some embodiments, the porous mercuryadsorptive material has a surface area of at least about 300 m²/g, andin other embodiments, the porous mercury adsorptive material furthercomprises a hydrophobicity enhancement agent. In particular embodiments,composition may have a halide concentration of greater than 0.15equivalents per 100 grams of the composition, and in certainembodiments, the at least one agent is from about 15 wt. % to about 70wt. % of the composition, and particular embodiments, the compositionmay be a dry admixture.

Certain embodiments are directed to a sorbent for the removal of mercuryfrom flue gas streams including a dry admixture of a porous mercuryadsorptive material and at least one agent selected from the groupconsisting of ammonium halides, amine halides quaternary ammoniumhalides, and combinations thereof. In some embodiments, the dryadmixture may have a mean particle diameter of less than about 12 μm. Inother embodiments, the dry admixture may have a mean particle diameterof less than about 10 μm, and in still other embodiments, the dryadmixture has a mean particle diameter of less than about 7 μm.

The porous mercury adsorptive material of embodiments may be acarbonaceous char, activated carbon, reactivated carbon, zeolite,silica, silica gel, alumina clay, or a combination thereof, and in someembodiments, the porous mercury adsorptive material may have a surfacearea of at least about 300 m²/g. In certain embodiments, the porousmercury adsorptive material may further include a hydrophobicityenhancement agent including, but not limited to, elemental halogensfluorine salts, organo-fluorine compounds, fluorinated polymers, andcombinations thereof. In other embodiments, the porous mercuryadsorptive material may further include one or more oxidants such as,but not limited to, halogen salts, and in some embodiments, the oxidantmay be greater than or equal to about 0.15 equivalents per 100 grams ofthe dry admixture.

In particular embodiments, the at least one agent may be ammoniumbromide. In some embodiments, the at least one agent may be about 10 wt.% or greater of the dry admixture. In other embodiments, the at leastone agent may be about 15 wt. % or greater of the dry admixture, and instill other embodiments, the at least one agent may be about 30 wt. % orgreater of the total sorbent. In some embodiments, the at least oneagent may be from about 15 wt. % to about 70 wt. % of the dry admixture.In other embodiments, the at least one agent may be from about 15 wt. %to about 50 wt. % of the dry admixture, and in still other embodiments,the at least one agent may be from about 20 wt. % to about 50 wt. % ofthe dry admixture. In certain embodiments, the at least one agent may befrom about 20 wt. % to about 40 wt. % of the dry admixture. In furtherembodiments, the at least one agent may be greater than or equal toabout 0.15 equivalents per 100 grams of the dry admixture.

Other embodiments are directed to methods for preparing a sorbent forthe removal of mercury from flue gas streams including the steps ofco-milling a dry porous mercury adsorptive material and at least one dryagent selected from the group consisting of ammonium halides, aminehalides, and quaternary ammonium halides to form a dry admixture havinga mean particle diameter of less than or equal to about 12 μm, and inparticular embodiments, the porous mercury adsorptive material and atleast one agent are not physically associated in the dry admixture. Insome embodiments, co-milling may be performed until the dry admixturehas a mean particle diameter of less than about 10 μm, and in otherembodiments, co-milling may be performed until the dry admixture has amean particle diameter of less than about 7 μm. The porous mercuryadsorptive material and various agents may be any of those agentsprovided in the above embodiments, and the admixture may include anyamount of the components as described above.

Still other embodiments include a method for removing mercury from fluegas streams containing sulfur trioxide (SO₃) including the steps ofinjecting into the flue gas stream a dry admixture of a porous mercuryadsorptive material and at least one dry agent selected from the groupconsisting of ammonium halides, amine halides, and quaternary ammoniumhalides, wherein the dry admixture has a mean particle diameter of lessthan or equal to about 12 μm. In some embodiments, the dry porousmercury adsorptive material and the at least one dry agent may becombined to form a dry admixture prior to being injected into the fluegas stream, and in other embodiments, the dry porous mercury adsorptivematerial and the at least one dry agent may be injected in to the fluegas stream separately. The porous mercury adsorptive material andvarious agents may be any of those agents described in the aboveembodiments, and the admixture may include any amount of the componentsas described above.

DESCRIPTION OF DRAWINGS

Not applicable.

DETAILED DESCRIPTION

Before the present compositions and methods are described, it is to beunderstood that this invention is not limited to the particularprocesses, compositions, or methodologies described, as these may vary.It is also to be understood that the terminology used in the descriptionis for the purpose of describing the particular versions or embodimentsonly, and is not intended to limit the scope of the present invention,which will be limited only by the appended claims. Unless definedotherwise, all technical and scientific terms used herein have the samemeaning as commonly understood by one of ordinary skill in the art.Although any methods and materials similar or equivalent to thosedescribed herein can be used in the practice or testing of embodimentsof the present invention, the preferred methods, devices, and materialsare now described. All publications mentioned herein are incorporated byreference in their entirety. Nothing herein is to be construed as anadmission that the invention is not entitled to antedate such disclosureby virtue of prior invention.

It must also be noted that as used herein and in the appended claims,the singular forms “a,” “an,” and “the” include plural reference unlessthe context clearly dictates otherwise. Thus, for example, reference to“a combustion chamber” is a reference to “one or more combustionchambers” and equivalents thereof known to those skilled in the art, andso forth.

As used herein, the term “about” means plus or minus 10% of thenumerical value of the number with which it is being used. Therefore,about 50% means in the range of 45%-55%.

Embodiments of the invention are directed to mercury sorbents havingenhanced mercury removal capabilities in flue gas streams containinghigh concentrations of acid gases. In various embodiments, the mercurysorbent may include a mercury adsorptive material and a acid gassuppression agent. In some embodiments, the mercury adsorptive materialmay be an adsorptive material, such as, a carbonaceous char or activatedcarbon that has been treated with one or more additives that enhance thehydrophobicity of the adsorbent material, and in other embodiments, themercury adsorptive material may be treated with a mercury adsorptionenhancing additive. In certain embodiments, acid gas suppression agentmay be any agent having an exceptionally high affinity, selectivity, andrapid kinetics for acid gases such as, for example, hydrochloric acid(HCl), hydrofluoric acid (HF), nitric oxide species (NO_(x)), or sulfuroxide species (SO_(x)). In certain embodiments, the sorbents of theinvention may include a porous mercury sorbent and a SO_(x) suppressionagent. Such sorbents may more effectively reduce the concentration ofmercury in flue gas streams into which they are added over mercuryadsorptive materials alone, in particular, when injected into flue gasstreams containing high concentrations of sulfur oxide species.

In general, mercury adsorptive materials such as activated carbon removecarbon adsorb mercury with less efficiency in flue gas streams havinghigh concentrations of acid gases and, in particular, sulfur oxidespecies, SO_(x), such as, SO₃ and/or SO₂, and other acid gases. Sulfurtrioxide, SO₃, is strongly adsorbed by activated carbon. Sulfur dioxide,SO₂, although less strongly adsorbed, can be oxidized to sulfur trioxideby oxygen in the flue gas in the presence of catalytic sites on theadsorbent surface. The overall effect of adsorption of these sulfuroxides precludes or strongly interferes with the adsorption of mercuryfrom the flue gas.

The adsorption of sulfur oxide species may be further enhanced in thepresence of water, which is commonly present in flue gas steams.Accordingly, increasing the inherent hydrophobicity of the adsorbentsurface or making it less receptive to moisture adsorption can limit theformation and retention of sulfur species on the carbon surface,preserving more of the adsorption pore volume for mercury removal. Thus,various embodiments of the invention include hydrophobic mercuryadsorptive materials. As used herein “hydrophobic” describes the forcesdriving a solute out of water (or other polar solvent) and the tendencyfor collection on surfaces or in non-polar solvents. In someembodiments, the hydrophobicity of the adsorbent may be enhanced toreduce SO_(x) adsorption to the adsorptive material, and in otherembodiments, the porous regions of the adsorptive material may betreated such that the kinetics of mercury removal are enhanced relativeto the kinetics of sulfur oxide removal.

The mercury adsorptive material of the sorbent composition of variousembodiments may include any material having an affinity for mercury. Forexample, in some embodiments, the mercury adsorptive material may be aporous sorbent having an affinity for mercury including, but not limitedto, activated carbon, reactivated carbon, zeolite, silica, silica gel,clay, and combinations thereof, and in particular embodiments, themercury adsorptive material may be activated carbon.

The mercury adsorptive material may have any mean particle diameter(MPD). For example, in some embodiments, the MPD of the mercuryadsorptive material may be from about 0.1 μm to about 100 μm, and inother embodiments, the MPD may be about 1 μm to about 30 μm. In stillother embodiments, the MPD of the mercury adsorptive material may beless than about 15 μm, and in some particular embodiments, the MPD maybe about 2 μm to about 10 μm, about 4 μm to about 8 μm, or about 5 μm orabout 6 μm. In certain embodiments, the mercury adsorptive materials mayhave an MPD of less than about 12 μm, or in some embodiments, less than7 μm, which may provide increased selectivity for mercury oxidation.

The mercury adsorptive materials of various embodiments may have aninherent hydrophobic character. For example, while activated carbon isinherently hydrophobic to the extent that it is wet more readily byorganic solvents than by water, all activated carbons are not completelyhydrophobic, and certain portions of the surface of activated carbonsmay exhibit partially hydrophilic characteristics. In some embodiments,the feedstock used to prepare the mercury adsorptive material may beselected from materials having fewer carbon-oxygen surface groups, whichprovide fewer active sites for carbon self-oxidation and fewer sites foroxidation of sulfur dioxide to sulfur trioxide. Such materials may beselected using, for example, a high peroxide index as described in U.S.Pat. Nos. 6,238,641 and 6,514,906, which are hereby incorporated byreference in their entireties.

In other embodiments, the mercury adsorptive material may be treated toenhance the hydrophobicity of the adsorptive materials with, forexample, one or more hydrophobicity enhancement agents that impede theadsorption and transport of water or other treatments of the sorbentthat achieve similar results. Embodiments are not limited to the type oftreated mercury adsorptive material or the means by which the mercuryadsorptive material has been treated with a hydrophobicity enhancementagent. For example, in some embodiments, the mercury adsorptive materialmay be treated with an amount of one or more elemental halogen that canform a permanent bond with the surface. The elemental halogen may be anyhalogen such as fluorine (F), chlorine (Cl), or bromine (Br), and incertain embodiments, the elemental halogen may be fluorine (F). In otherembodiments, the mercury adsorptive material may be treated with ahydrophobicity enhancement agent such as a fluorine salt,organo-fluorine compound, or fluorinated polymer, such as, TEFLON®. Insuch embodiments, treatment may be effectuated by grinding the mercuryadsorptive material with the organo-fluorine compound or fluorinatedpolymer. In still other embodiments, carbon sorbents used as the mercuryadsorptive material may be treated with mineral acids such as but notlimited to, hydrochloric acid, nitric acid, boric acid, and sulfuricacid, under high temperature, e.g., greater than about 400° C. orgreater than 600° C. or greater than 800° C. The concentration of theacid is not critical to such treatments and concentrations as low as 1.0percent by weight or less may be used. Without wishing to be bound bytheory, such treatment may enhance hydrophobicity and decreased activityfor the catalytic oxidation of sulfur dioxide to sulfuric acid in thepresence of oxygen and water. Evidence of such treatments can be foundin a high contact pH and a reduced tendency for the carbon alone todecompose hydrogen peroxide when compared to the same carbon withoutsuch treatments.

In still other embodiments, any of the adsorptive materials describedabove may be treated with one or more mercury removal agents oroxidizing agents that enhance mercury adsorption. For example, in someembodiments, the mercury removal agent may be a halogen salt includinginorganic halogen salts, which for bromine may include bromides,bromates, and hypobromites, for iodine may include iodides, iodates, andhypoiodites, and for chlorine may be chlorides, chlorates, andhypochlorites. In certain embodiments, the inorganic halogen salt may bean alkali metal or an alkaline earth element containing halogen saltwhere the inorganic halogen salt is associated with an alkali metal suchas lithium, sodium, and potassium or alkaline earth metal such asmagnesium, and calcium counterion. Non-limiting examples of inorganichalogen salts including alkali metal and alkali earth metal counterionsinclude calcium hypochlorite, calcium hypobromite, calcium hypoiodite,calcium chloride, calcium bromide, calcium iodide, magnesium chloride,magnesium bromide, magnesium iodide, sodium chloride, sodium bromide,sodium iodide, ammonium chloride, ammonium bromide, ammonium iodide,potassium tri-chloride, potassium tri-bromide, potassium tri-iodide, andthe like. In particular embodiments, the halogen salt may be a brominesalt, such as calcium bromide (CaBr₂). In such embodiments, the oxidantcontent may be near to or above about 0.15 equivalents per 100 grams ofabsorptive material, wherein one equivalent of oxidant is defined as theamount required to react with one mole of electrons in a redox reaction.

In various embodiments, treatment of the mercury adsorptive materialwith a halogen salt or other mercury removal agent may be effectuated bygrinding the mercury adsorptive material with the halogen salt or othermercury removal agent. For example, in some embodiments, increasedselectivity for mercury adsorption over sulfur trioxide adsorption maybe provided by co-milling activated carbon with a halide salt to an MPDof less than about 10 μm or less than about 7 μm. Although not wishingto be bound by theory, the small MPD may improve the selectivity ofmercury adsorption as the halide effectively oxidizes the mercury andthe alkalinity interferes with the adsorption of the sulfur trioxide.

Certain embodiments of the invention include methods for treating anadsorptive material by grinding the adsorptive material with a halogensalt or other mercury removal agent and an organo-fluorine compound orfluorinated polymer. For example, in some embodiments, the mercuryadsorptive material may be treated with both a halogen salt or othermercury removal agent and a solid hydrophobicity enhancement agent suchas an organo-fluorine compound or a fluorinated polymer, and in suchembodiments, the adsorptive material may be co-ground with both agents.

In some embodiments, mercury removal may be further enhanced bycombining the adsorptive materials described above with one or moresecondary agents having high affinity, high selectivity, and rapidkinetics for acid gas removal, which may be collectively referred toherein as “acid gas suppression agents” or “acid gas suppressors.” Insome embodiments, the acid gas agents may not be physically incorporatedinto and within the adsorptive material itself. Rather, the acid gasagents may be provided as a separate component of the mercury sorbentthat is blended with the adsorptive agent; therefore, the maximum porespace for mercury reaction and adsorption can be maintained on theadsorptive material. In certain embodiments, the acid gas suppressionagents may have high affinity, high selectivity, and rapid kinetics forsulfur reactive species such compositions are referred to herein as“SO_(x) suppression agents” or “SO_(x) suppressors.” The resultingmercury sorbent thus includes an adsorptive material and one or moreSO_(x) suppression agents. Any type of SO_(x) suppression agent known inthe art may be used in the mercury sorbent of various embodiments. Forexample, the SO_(x) suppression agent may be an oxidizing agent,alkaline agent, dual-function agent having both alkalinity and oxidationcapabilities, or adsorptive agent treated to specifically adsorb sulfuroxides.

In some embodiments, the acid gas or SO_(x) suppression agent may be analkaline agent. Numerous alkaline agents are known in the art andcurrently used to remove sulfur oxide species from flue gas and any suchalkaline agent may be used in the invention. For example, in variousembodiments, the alkaline additive may be alkali oxides, alkaline earthoxides, hydroxides, carbonates, bicarbonates, phosphates, silicates,aluminates, and combinations thereof, and in certain embodiments, thealkaline additive may be calcium carbonate (CaCO₃; limestone), calciumoxide (CaO; lime), calcium hydroxide (Ca(OH)₂; slaked lime); magnesiumcarbonate (MgCO₃; dolomite), magnesium hydroxide (Mg(OH)₂), magnesiumoxide (MgO), sodium carbonate (Na₂CO₃), sodium bicarbonate (NaHCO₃),trisodium hydrogendicarbonate dihydrate (Na₃H(CO₃)₂.2H₂O; trona), andcombinations thereof. In various embodiments, the alkalinity agent maybe provided at a concentration greater than or equal to about 0.15equivalents per 100 grams of absorptive material, wherein one equivalentof the alkaline agent is defined as the amount required to produce onemole of hydroxyl ions or to react with one mole of hydrogen ions. Inparticular embodiments, such alkaline agents may have a relatively highsurface area such as, for example, above 100 m²/g for neat materials.High surface area materials may provide improved kinetics andcapabilities for acid gas or SO_(x) mitigation while complementinghalogen compounds and other added oxidants to provide oxidation ofelemental mercury. Because alkaline agents are highly polar materialsthat may associate and bond with water, in various embodiments, alkalineagents may be combined with the primary mercury sorbent as a physicaladmixture and may not generally be present on the sorbent surface orcontained within the sorbent pore structure.

In further embodiments, the acid gas or SO_(x) suppression agent is adual-function agent having both acid gas adsorption capacity and mercuryoxidation activity on the same agent. In some embodiments, the dualfunction agent may be a readily dissociable additive with an oxidizingcomponent and alkaline component. Examples of such agents include anon-metal cation and a halide. In some embodiments, such compounds mayinclude a halogen and a source of nitrogen having an oxidation state of−3. Various such nitrogen sources are known in the art and can include,for example, ammonium, ammonia, amines, amides, imines, quaternaryammonium, and the like. In certain embodiments, the agent may be, forexample, ammonium halide, such as, ammonium iodide, ammonium bromide, orammonium chloride, an amine halide, a quaternary ammonium halide, or anorgano-halide and combinations thereof. These agents can be combinedwith a porous mercury adsorbent to provide the compositions and sorbentsof the invention.

In such embodiments, an ammonium halide such as, for example, ammoniumbromide (NH₄Br), can react with the sulfur oxides in the flue gas toform ammonium sulfate ((NH₄)₂SO₄) or ammonium sulfite ((NH₄)₂SO₃) andfree bromine. The bromine liberated by this reaction is notsubstantially adsorbed by the activated carbon and is available tooxidize mercury in the flue gas, which is readily adsorbed by adsorptivematerial, although secondary oxidation pathways such as those affordedby the Deacon reaction may also be effective. Without wishing to bebound by theory, these dual-function additives may have higher vaporpressures and lower decomposition temperatures compared to alkali oralkaline earth metal halide salts that are commonly provided withmercury adsorbents, which are provided solely for oxidation of mercury.Thus, the dual-function additives may provide effective mercuryoxidation together with improved kinetics for SO_(x) suppression andsulfur trioxide adsorption.

Acid gas or SO_(x) suppression agents having a dual-function agent maybe prepared by any method known in the art. For example, in someembodiments, one or more ammonium halide, amine halide, or quaternaryammonium halide may be prepared independently and either combined with amercury adsorptive agent or combined with an adsorptive material suchas, for example, activated carbon, under conditions that do not allowthe dual-function agent to impregnate and bind to the adsorptive agent.In other embodiments, an ammonium, amine, or quaternary ammoniumcontaining compound may be combined with an adsorptive materialsimultaneously with an elemental halogen or decomposable halogencompound. In particular embodiments, the acid gas or SO_(x) suppressionagent may include a dual-function agent having a surface area greaterthan about 50 m²/g or greater than about 100 m²/g for the neatmaterials. In particular embodiments, the dual-function agent may beprovided at a concentration of greater than or equal to about 0.15equivalents per 100 grams of absorptive material based on either theoxidant or alkalinity contributions. In various embodiments, the dualfunction agents such as ammonium halides, amine halides, or quaternaryammonium halides may be combined with the primary mercury sorbent as adry physical admixture and may not generally be present on the sorbentsurface or contained within the sorbent pore structure.

The acid gas or SO_(x) suppression agent is provided in the mercurysorbent of various embodiments to suppress acid gases, such as,hydrochloric acid, hydrofluoric acid, nitric oxide species, or sulfurdioxide and sulfur trioxide adsorption to the adsorptive material whichwould reduce the adsorption of mercury to the adsorptive material. Theadsorptive material and the acid gas or SO_(x) suppression agent may becombined in any ratio which achieves suppression of acid gas and SO_(x)species while providing sufficient mercury removal. For example, in someembodiments, the adsorptive material to acid gas or SO_(x) suppressionagent ratio may be from about 1:1, about 1:5, about 1:10, about 1:25,about 1:50, about 75:1, or about 1:100. In other embodiments, theadsorptive material to acid gas or SO_(x) suppression agent ratio may befrom about 5:1, about 10:1, about 25:1, about 50:1, about 75:1, or about100:1. In other embodiments, the acid gas or SO_(x) suppression agentmay make up about 10 wt. % or greater or about 15 wt. % or greater ofthe total sorbent, and in still other embodiments, the acid gas orSO_(x) suppression agent may make up about 30 wt. % or greater, 40 wt. %or greater, 50 wt. % or greater, 60 wt. % or greater, or 70 wt. % orgreater of the total sorbent. In other embodiments, the dual functionagents may be combined with other agents such as, for example, halidesalts, halide metal salts, alkaline agents, and the like to prepare acomposition or sorbent encompassed by the invention.

In particular embodiments, the sorbent may include a mercury adsorptivematerial such as activated carbon or treated activated carbon and anagent such as ammonium halide, amine halide, or quaternary ammoniumhalide, for example, ammonium bromide. In some such embodiments, theammonium bromide may be provided about 10 wt. % or greater or about 15wt. % or greater, about 20 wt. % or greater, about 25 wt. % or greater,about 30 wt. % or greater, about 40 wt. % or greater of the totalsorbent. In other embodiments, the sorbent may include from about 10 wt.% to about 70 wt. %, about 10 wt. % to about 60 wt. %, or about 10 wt. %to about 50 wt. % SO_(x) suppression or about 15 wt. % to about 70 wt.%, about 15 wt. % to about 60 wt. %, or about 15 wt. % to about 50 wt. %SO_(x) suppression agent or about 20 wt. % to about 70 wt. %, about 20wt. % to about 60 wt. %, or about 20 wt. % to about 50 wt. % SO_(x)suppression agent. Without wishing to be bound by theory, improved acidgas and SO_(x) suppression may allow for improved mercury adsorption bythe mercury adsorptive agent, and increasing the concentration of theacid gas or SO_(x) suppression agent, and in particular a ammoniumhalide, amine halide, or quaternary ammonium halide such as, forexample, ammonium bromide, may improve mercury adsorption beyondcurrently available adsorbents thereby providing a mercury adsorbentthat includes, for example, low activated carbon content but thatremoves mercury from flue gas streams as effectively as high activatedcarbon content sorbents. Low activated carbon sorbents may provideimproved stability when used in, for example, cement manufacturing.

The compositions of various embodiments described above may allow for ahigher percentage of active halide and alkaline agents to be included inthe injected sorbent. Mercury adsorbents that are impregnated with anadditive by treating with an aqueous solution of the additive, forexample, commercial brominated carbon sorbents, especially thoseimpregnated with elemental bromine, can only retain a small percentageof the additive on the surface of the adsorbent, and impregnation tendsto clog the pores of porous mercury adsorbents reducing the surface areaavailable for mercury adsorption. In contrast, the percentage of activehalide and acid gas or SO_(x) suppression agent in a dry mixture may begreater than about 10 wt. %, greater than about 15 wt. %, greater thanabout 20 wt. %, or greater than about 30 wt. % and up to about 50 wt. %,up to about 60 wt. %, or up to about 70 wt. % without exhibiting areduction in mercury adsorption efficiency.

Moreover, the sorbents of various embodiments exhibit improved stabilityduring manufacture, storage, and injection than currently availableimpregnated sorbents. For example, producing acid gas or SO_(x)suppression agents having a mean particle diameter of less than about 15μm or 20 μm using any of the acid gas or SO_(x) suppression agentsdescribed herein is difficult. Moreover, all of the acid gas or SO_(x)suppression agents are somewhat hygroscopic, and ammonium halide, aminehalide, or quaternary ammonium halide acid gas or SO_(x) suppressionagents readily absorb water. Rapid moisture pickup causes substantialre-agglomeration making maintenance of acid gas or SO_(x) suppressionagents at mean particle diameters of less than about 15 μm difficult.Without wishing to be bound by theory, re-agglomeration may be reducedas the result of the mercury adsorbent acting as separators andcompeting desiccants reducing the amount of moisture in the dry mixtureand allowing long term storage and maintenance of acid gas or SO_(x)suppression agents with mean particle diameters of less than about 12μm. Reduction in particles size may also provide more rapid andselective kinetics allowing improved synergistic effects.

In addition, at elemental bromine loadings above 10 wt % to 15 wt %, theequilibrium vapor phase concentrations under ambient conditions may riseabove safe and acceptable threshold limit values (0.66 mg/m³ TWA; 2.0mg/m³ STEL), creating problems in handling and use, and SO_(x)suppression agents such as ammonium halide, amine halide, or quaternaryammonium halide may provide fire retardant properties that reduce selfheating and combustion associated with metal halide containing sorbentsin which the metal cation can catalyze the oxidation of the carbon.

The adsorptive material and the SO_(x) suppression agent may be combinedby any method. For example, in some embodiments, the adsorptive materialand the SO_(x) suppression agent may be combined by blending or mixingthe materials into a single mercury sorbent that can then be injectedinto a flue gas stream. In other embodiments, combining may occur duringuse such that the adsorptive material and the SO_(x) suppression agentmay be held in different reservoirs and injected simultaneously into aflue gas stream.

In certain embodiments, the absorptive material and the SO_(x)suppression agent may be co-milled. For example, in various embodiments,a porous absorptive material and a acid gas or SO_(x) suppression agentmay be combined and co-milled or sized to about the same particle sizedistribution which in some embodiments, may be a mean particle diameterof less than or equal to about 12 μm less than or equal to about 10 μm,or less than about 7 μm. Without wishing to be bound by theory, reducingthe mean particle diameter of the sorbent, combined with localized acidgas or SO_(x) suppression added to the sorbent, but not contained withinthe sorbent pore structure, has been found to be surprisingly effectivein facilitating rapid and selective mercury adsorption despite sulfurtrioxide concentrations that are orders of magnitude higher than themercury levels in the flue gas. This effect has been shown particularlyeffective when all of components of the sorbent are combined andco-milled or otherwise sized to a mean particle diameter of less than orequal to about 12 μm. Co-milling may be carried out by any means. Forexample, in various embodiments, the co-milling may be carried out usingbowl mills, roller mills, ball mills, jet mills or other mills or anygrinding device known to those skilled in the art for reducing theparticle size of dry solids.

The sorbent of such embodiments may include any of the absorptivematerials described above, any additive described above, and any acidgas or SO_(x) suppression agent described above. In certain embodiments,the absorptive material may be an activated carbon or reactivatedcarbon. In some embodiments, the additive provides rapid ancillaryoxidation of elemental mercury in the gas stream such as, for example, ahalide compound, and in particular embodiments, the halide compound maybe less stable at elevated temperatures than alkali or alkaline-earthmetal analogs. In certain embodiments, the SO_(x) suppression agent maybe a dual function agent providing both oxidation and alkalinity suchas, for example, ammonium halides, amine halides, and quaternaryammonium halides.

Further embodiments are directed to methods for removing mercury fromflue gas by injecting a mercury adsorbent including a mercury adsorptivematerial and an acid gas or SO_(x) suppression agent into a flue gasstream. While such compositions may be particularly effective in fluegas streams having high SO_(x), and in particular, high SO₃,concentrations, the sorbents described herein may be used to adsorbmercury in any flue gas streams regardless of the SO₃ concentration. Forexample, the sorbents of various embodiments may be used in flue gasstreams having no or extremely low SO₃ content or flue gas streamscontaining high concentrations of other acid gases such as HCl, HF, orNO species. In some embodiments, the mercury adsorptive material and theacid gas or SO_(x) suppression agent may be combined prior to injectioninto the flue gas stream by, for example, mixing or blending, themercury adsorptive material with the acid gas or SO_(x) suppressionagent. In other embodiments, the mercury adsorptive material and theacid gas or SO_(x) suppression agent may be injected separately into theflue gas stream and combined in the flue gas stream itself In suchembodiments, the acid gas or SO_(x) suppression agent may adsorb sulfuroxide species such as sulfur trioxide and sulfur dioxide reducing thelikelihood that such sulfur oxide species will adsorb to the mercuryadsorptive agent. The increased availability for mercury adsorption ofsurface area on the mercury adsorptive agent may thus increase mercuryadsorption.

EXAMPLES

Although the present invention has been described in considerable detailwith reference to certain preferred embodiments thereof, other versionsare possible. Therefore the spirit and scope of the appended claimsshould not be limited to the description and the preferred versionscontained within this specification. Various aspects of the presentinvention will be illustrated with reference to the followingnon-limiting examples.

Example 1

Power Plant 1—Three mercury sorbents were tested at a power plantburning a 25% PRB/75% CAPP coal mixture. Sorbent C was prepared bydry-mixing and co-grinding an activated carbon with about 15 wt. %bromide as ammonium bromide to an MPD of about 6 μm. Sorbent B wasprepared by dry-mixing and co-grinding an activated carbon with about 15wt % calcium as calcium oxide and about 5 wt. % bromide as ammoniumbromide to an MPD of about 6 μm. These sorbents were compared to acommercial brominated carbon, Sorbent H, having an MPD not less thanabout 12 μm and containing no more than about 10 wt. % bromide as abromide salt. The flue gas at this plant contained about 400 to about600 ppmv sulfur dioxide and about 2 to about 10 ppmv sulfur trioxide atthe SCR, selective catalytic reduction, outlet. When injected at the airpre-heater outlet at about 9 to about 9.5 lbs/MMacf the commercialsorbent, Sorbent H, achieved mercury removal levels of about 55% versusabout 77% for Sorbent B and 85% for Sorbent C.

Example 2

At Power Plant 2 burning 100% CAPP coal and generating greater than 10ppmv of sulfur trioxide, Sorbent C, prepared as described in Example 1was tested for mercury removal against Sorbent H, as described inExample 1. To achieve and maintain mercury removal levels around orabove 90%, Sorbent H required injection rates of about 6 lbs/MMacfversus about 2 lbs/MMacf for Sorbent C.

Example 3

At Power Plant 3 where sulfur trioxide levels could be controlled andvaried by the deliberate addition of such to the flue gas stream,several mercury removal sorbents were tested at different sulfurtrioxide concentrations. Sorbent C was prepared by dry-mixing andco-grinding activated carbon with about 15 wt. % bromide as ammoniumbromide to an MPD of about 6 μm. This sorbent was tested against SorbentM, a commercial brominated carbon sorbent having an MPD not less thanabout 12 μm and containing not more than about 10 wt. % bromide as abromide salt. Sorbent C was also tested against Sorbent N, anothercommercial brominated carbon sorbent believed to be prepared from alignite-based activated carbon impregnated internally with about 3 toabout 6 wt % bromide as sodium bromide and ground to an MPD of about 14to about 16 μm.

With no sulfur trioxide in the flue gas, Sorbent C gave nearly 100%removal at a sorbent injection rate of 10 lbs/MMacf versus 85% and 76%removal for Sorbents M and N, respectively, at the same injection rate.At 10.3 ppmv sulfur trioxide and the same sorbent injection rate,Sorbent C gave about 94% mercury removal, while commercial brominatedsorbents M and N declined to about 65% and 62%, respectively. At 20.3ppmv of sulfur trioxide and an injection rate of 10 lbs/MMacf, Sorbent Cachieved about 78% mercury removal, versus about 53% removal for both ofthe commercial brominated sorbents M and N. Adding 20 lbs/hr of trona at25.3 ppmv sulfur trioxide reduced the Sorbent C injection rate to about7 lbs/MMacf to maintain about 78% mercury removal, while about 8lbs/MMacf were required for Sorbent M to maintain about 53% mercuryremoval. The sulfur trioxide levels reported in this example were allmeasured in front of the air preheater.

Example 4

Power Plant 3—Sorbent C, which includes about 15 wt. % bromine asammonium bromide dry-mixed and co-ground with activated carbon to an MPDof about 6 μm, and Sorbent D, which includes about 36 wt. % bromine asammonium bromide dry-mixed and co-ground with activated carbon to an MPDof about 6 μm, were tested against Sorbent N, a commercial brominatedcarbon sorbent believed to be made from a lignite activated carbonimpregnated internally with about 3 to about 6 wt. % bromine as sodiumbromide and ground to an MPD of about 14 to about 16 μm. At thiscoal-fired boiler, sulfur trioxide (SO₃) could be added to the flue gasin a controlled manner. At an SO₃ concentration of about 10 ppmv,injection rates for each sorbent were increased until 90% mercuryremoval had been achieved. For Sorbent N, between 50 and 60 lbs/min wererequired to meet the treatment objective, versus about 20 to about 25lbs/min for Sorbent C and about 12 to about 15 lbs/min for Sorbent D,demonstrating the value in high SO₃ streams of both the higheralkalinity/halide content of the co-ground dual-function additive aswell as smaller sorbent mean particle diameters. The sulfur trioxidelevels reported in this example were all measured in front of the airpreheater.

Example 5

At Power Plant 3—Sorbent C, containing about 15 wt. % bromide asammonium bromide dry-mixed and co-milled with activated carbon to an MPDof about 6 μm, was tested in a flue gas stream containing about 9 ppmvsulfur trioxide. At an injection rate of about 24 pounds/hour, Sorbent Cachieved about 92% mercury removal compared to only about 79% removal atan injection rate of 40 pounds/hour for Sorbent E, a sorbent comparableto Sorbent C in all respects but for a 15 wt. % bromine content providedby sodium bromide instead of ammonium bromide. This example illustratesthe added utility for mercury removal afforded by the alkaline componentof the dual-function ammonium bromide additive independent of othersorbent parameters.

Example 6

At Power Plant 4—Sorbent C, an activated carbon dry-mixed with about 15wt. % bromine as ammonium bromide and co-ground to an MPD of about 6 μm,was tested at another power plant against Sorbent N, a commercialbrominated carbon sorbent believed to made from a lignite activatedcarbon impregnated internally with about 3 to about 6 wt. % bromine assodium bromide ground to an MPD of about 14 to about 16 μm. The flue gasof this boiler contained about 4 ppmv of sulfur trioxide.

When injected at the air pre-heater outlet at a rate of about 6lbs/MMacf, Sorbent C achieved about 72% mercury removal versus about 42%for Sorbent N, showing the benefits of a smaller sorbent MPD combinedwith a higher sorbent content of a dual-functioning agent for mercuryoxidation and SOx suppression.

Example 7

At Power Plant 5 burning a high percentage of a low-sulfur SouthAmerican coal with a lower percentage of a high-sulfur domestic coal,about 61% mercury removal was achieved using Sorbent C, including about15 wt. % bromine as ammonium bromide dry-mixed and co-ground withactivated carbon to an MPD of about 6 μm, and injected at the airpre-heater inlet at a rate of about 8 lbs/MMacf for a 70/30 blend,respectively, of the low and high sulfur content domestic coals. The SO₃in the flue gas at the point of injection was measured to be about 8 toabout 9 ppmv. Under the same conditions, Sorbent D, a sorbent comparableto Sorbent C but containing about 36 wt. % bromine as ammonium bromide,gave about 81% mercury removal under the same conditions, demonstratingthe value of the higher localized alkalinity and halide levels affordedby higher levels of the dual-function additive.

Example 8

Power Plant 5—Three mercury sorbents were tested at Power Plant 5 firedwith a coal blend of 20% PRB and 80% of another low-sulfur coal. Thesulfur trioxide levels in the flue gas ranged from about 0.5 ppmv tonon-detectable. Sorbent C was prepared by dry-mixing and co-grinding anactivated carbon with about 15 wt. % bromide as ammonium bromide to anMPD of about 6 μm. Sorbent D was prepared similarly, but contained about36 wt. % bromide as ammonium bromide. These sorbents were compared toSorbent M, a commercial brominated carbon having an MPD not less thanabout 12 μm and a bromide content of not more than about 10 wt. % as abromide salt. When injected at the air pre-heater inlet, about 8lbs/MMacf were required to achieve about 90% mercury removal withSorbent M, versus about 5.5. lbs/MMacf for sorbent C, and about 3lbs/MMacf for Sorbent D. These data demonstrate the significantperformance gains that can be obtained with bromide levels above about10 wt. % and MPD reductions below about 12 μm, even in flue gas streamswith little to no sulfur trioxide.

Example 9

At Power Plant 6 containing 1 to 2 ppmv of SO₃ in the flue gas, SorbentC, containing about 15 wt. % bromine as ammonium bromide dry-mixed andco-ground with activated carbon to an MPD of about 6 μm, was testedagainst Sorbent M, a commercially-available brominated carbon sorbenthaving an MPD not less than about 12 μm and containing not more thanabout 10 wt. % bromine as a bromide salt, and against Sorbent K, acommercially-available brominated carbon sorbent believed to be derivedfrom a lignite or brown coal. When injected at the electrostaticprecipitator (ESP) inlet, Sorbent K required an injection rate of about9 lbs/MMacf to achieve 90% mercury removal, versus about 4.5 lbs/MMacffor Sorbent M and about 2 lbs/MMacf or less for Sorbent C.

Example 10

Power Plant 7—Five mercury sorbents were prepared by dry-mixing andco-grinding activated carbon with either 10 or 20 wt. % bromide aseither ammonium or sodium bromide: Sorbent P contained about 10 wt. %bromide as ammonium bromide co-ground to about a 6 μm MPD; Sorbent Qcontained about 10 wt. % bromide as ammonium bromide co-ground to abouta 16 μm MPD; Sorbent S contained about 10 wt. % bromide as sodiumbromide co-ground to about 16 μm MPD; Sorbent R contained about 20 wt %bromide as ammonium bromide co-ground to about a 16 μm MPD; and SorbentT contained about 20% bromide as ammonium bromide co ground to about 6μm. Sorbents P, R, Q, and S were tested for mercury removal at injectionrates of about 100 lbs/hour under comparable conditions at a Power Plant7 burning PRB coal and containing little sulfur trioxide in the fluegas.

Sorbents P, R, Q, and S achieved mercury removals of about 77%, 71%,66%, and 53%, respectively. Sorbent R versus sorbent Q demonstrates theadvantages of bromide levels above about 10 wt. % as ammonium bromide ata given MPD, while Sorbent P versus Sorbent Q demonstrates theadvantages of an MPD below about 10 μm at a given bromide level. SorbentQ versus Sorbent S demonstrates the advantages of using ammonium bromideover metal bromide salts, even in flue gas streams containing littlesulfur trioxide.

In a second test at Power Plant 7, Sorbent T was compared to Sorbent Pand Sorbent Q to determine the amount of sorbent required to remove 90%of the mercury. Sorbent T required approximately 270 lbs/hour, Sorbent Prequired approximately 320 lbs/hour, and Sorbent Q required over 420lbs/hour to achieve 90% removal.

While presently preferred embodiments of the invention have beendescribed, it is to be understood that the detailed embodiments arepresented for elucidation and not limitation. The invention may beotherwise varied, modified or changed within the scope of the inventionas defined in the appended claims.

1. A method for removing mercury from flue gas streams containing sulfurtrioxide (SO₃) comprising injecting into the flue gas stream a dryadmixture of a porous mercury adsorptive material and at least one dryagent selected from the group consisting of ammonium halides, aminehalides, and quaternary ammonium halides, wherein the dry admixture hasa mean particle diameter of less than or equal to about 12 μm.
 2. Themethod of claim 1, wherein the porous mercury adsorptive material andthe at least one dry agent are combined to form a dry admixture prior tobeing injected into the flue gas stream.
 3. The method of claim 1,wherein the porous mercury adsorptive material and the at least one dryagent are injected in to the flue gas stream separately.
 4. The methodof claim 1, wherein the at least one dry agent is ammonium bromide. 5.The method of claim 1, wherein the at least one dry agent is about 10wt. % or greater of the dry admixture.
 6. The method of claim 1, whereinthe at least one dry agent is about 15 wt. % or greater of the dryadmixture.
 7. The method of claim 1, wherein the at least one dry agentis about 30 wt. % or greater of the dry admixture.
 8. The method ofclaim 1, wherein the at least one dry agent is from about wt. % to about70 wt. % of the dry admixture.
 9. The method of claim 1, wherein the atleast one dry agent is from about wt. % to about 50 wt. % of the dryadmixture.
 10. The method of claim 1, wherein the at least one dry agentis from about 20 wt. % to about 50 wt. % of the dry admixture.
 11. Themethod of claim 1, wherein the at least one dry agent is from about 20wt. % to about 40 wt. % of the dry admixture.
 12. The method of claim 1,wherein the porous mercury adsorptive material is selected from thegroup consisting of carbonaceous char, activated carbon, reactivatedcarbon, zeolite, silica, silica gel, alumina clay, and a combinationthereof.
 13. The method of claim 1, wherein the porous mercuryadsorptive material has a surface area of at least about 300 m²/g. 14.The method of claim 1, wherein the porous mercury adsorptive materialfurther comprises a hydrophobicity enhancement agent.
 15. The method ofclaim 14, wherein the hydrophobicity enhancement agent is selected fromthe group consisting of elemental halogens fluorine salts,organo-fluorine compounds, fluorinated polymers, and combinationsthereof.
 16. The method of claim 1, wherein the porous mercuryadsorptive material further comprises one or more oxidants.
 17. Themethod of claim 16, wherein the one or more oxidants is a halogen salt.18. The method of claim 16, wherein the oxidant comprises greater thanor equal to about 0.15 equivalents per 100 grams of the admixture. 19.The method of claim 1, wherein the composition has a mean particlediameter of less than about 10 μm.
 20. The method of claim 1, whereinthe composition has a mean particle diameter of less than about 7 μm.